The following is an extensive catalogue of the variants and specific unique elements of each variant and/or design stage of the Boeing B-17 Flying Fortress heavy bomber. For a broader article on the history of the B-17, see B-17 Flying Fortress.

The Boeing 299 was the original bomber design made by Boeing to fulfill a request by the United States Army Air Corps for a bomber capable of carrying 2,000 lb (907 kg) of bombs 2,000 mi (3,218 km) at 200 mph (322 km/h). In 1935, the Boeing 299 competed with several entries by other companies at an evaluation at Wright Field.

On its flight from Seattle to Wright Field for the competition, the 299 set a nonstop speed record of 252 mph (406 km/h). Though it crashed to its destruction on takeoff during a demonstration, the crash was due to ground-crew error, not from any flaw in the airplane. Despite the crash (and more important, its much higher cost per unit), Air Corps leaders were impressed by the 299. Boeing was awarded with a development contract. The aircraft has been since been referred to as the XB-17 but the designation is not contemporary or official.

Though still enthusiastic about the Boeing design, the Army Air Corps decided to cut its order for service test YB-17s from 65 to 13 after the crash of the original Model 299. On November 20, 1936, the Army Air Corps changed the source of funding from normal funding to 'F-1' procurement[clarification needed], and redesignated the plane Y1B-17 before it even flew.

Unlike its predecessor, which had used Pratt & Whitney R-1690 Hornet radial engines, the Y1B-17 used the Wright R-1820 Cyclone that would become standard on the B-17. Several changes were made in the armament, and the crew was reduced from seven to six. Most changes were minor: the most notable was switching from double-wishbone to single-arm landing gear.

On December 7, 1936, five days after the first flight of the Y1B-17, the brakes on the aircraft fused during landing, and it nosed over. Though damage was minimal, the cumulative impact of this event and the crash of the Model 299 triggered a Congressional investigation. After the crash the Army Air Corps was put in notice: another crash would mean the end of the program.

Though they were meant for testing, the commander of Army General Headquarters (Air Force), Major General Frank Andrews, decided to assign twelve Y1B-17s to the 2nd Bomb Group at Langley Field, Virginia. Andrews reasoned that it was best to develop heavy bombing techniques as quickly as possible. Of the thirteen built, one was used for stress testing.

In 1937, the twelve Y1B-17s with the 2nd Bombardment Group at Langley Field represented the entire American fleet of heavy bombers. Most of the time spent with the planes entailed eliminating problems with the aircraft. The most important development was the use of a checklist, to be reviewed by the pilot and copilot before takeoff. It was hoped that this system would prevent accidents similar to that which led to the loss of the 299.

In May 1938, the Y1B-17s (now redesignated B-17) of the 2nd Bombardment Group, led by the lead plane's navigator Curtis LeMay, took part in a demonstration in which they intercepted the Italian liner Rex. Coming into contact with the liner while it was still 610 mi (982 km) out at sea, the demonstration was meant to prove the range and navigational superiority of the B-17. It also showed that the bomber would be an effective tool for attacking an invasion force before it could reach the United States. The Navy was furious about Army intrusion into their mission, and forced the War Department to issue an order restricting the Army Air Corps from operating more than a hundred miles from the American coast.

After three years of flight, no serious incidents occurred with the B-17s. In October 1940, they were transferred to the 19th Bombardment Group at March Field.

The aircraft that became the sole Y1B-17A was originally ordered as a static test bed. However, when one of the Y1B-17s survived an inadvertent violent spin during a flight in a thunderhead, Air Corps leaders decided that the plane was exceptionally robust and that there would be no need for static testing. Instead, it was used as a testbed for engine types. After studying a variety of configurations, use of a ventral-nacelle-mount turbocharger position was settled on for each engine. This General Electric-manufactured turbocharger became standard on the first production model, and allowed it to fly higher and faster than the Y1B-17. When testing was complete the Y1B-17A was renamed the B-17A.

The B-17B (299M) was the first production model of the B-17 and was essentially a B-17A with a larger rudder, larger flaps, and a redesigned nose and 1,200 hp (895 kW) R-1820-51 engines. The small gun turret in the upper nose blister was replaced with a .30 in (7.62 mm) machine gun, its barrel run through a flexible ball socket; the separate bombardier's window was replaced with a ten-panel, well framed "glass nose" for bomb-aiming, used through to the E-series airframes. During Army Air Corps service, its bulged machine gun blisters were replaced with the more aerodynamic flush side windows used on the B-17C/D

In the interior of the plane, crew members were rearranged. Many internal systems were revised. Most notable of these was a switch from pneumatic to hydraulicbrakes.

In October 1942 all planes of the B-17B designation were redesignated RB-17B, R- indicating 'restricted'. The RB-17B was used for training, transport, messenger, and liaison duties; it was in effect a designation of obsolescence.

The "B" series made its maiden flight on June 27, 1939. 39 were built in a single production run, but Army Air Corps serial numbers were scattered over several batches. This was because of limited funding: the Army Air Corps could only buy a few B-17Bs at a time.

The B-17C was a B-17B with a number of improvements, including more powerful R-1820-65 engines. To boost crew safety, the waist-mounted gun blisters were replaced with teardrop-shaped sliding panels flush with the fuselage, and the ventral gun blister by a metal housing dubbed a 'bathtub turret', similar in appearance and general location on the fuselage, to the Bola ventral gondola then being used on the He 111. Some of the most important additions were self-sealing fuel tanks and defensive armor.

With the passage of the Lend-lease Act in 1941, the Royal Air Force began clamoring[citation needed] for use of the B-17. At that time, the Army Air Corps was suffering from shortages of the B-17, but hesitantly[citation needed] agreed to provide 20 planes to the RAF. Though the Army Air Corps did not consider the B-17C ready for combat, it was desperately needed in Britain. They were modified Boeing production B-17C, given the company designation Model 299T. The modifications were the addition of self-sealing tanks and replacement of the single nose gun with 0.5 inch Brownings.[1]

The twenty planes were placed immediately into frontline service as the Fortress Mk I.

In Britain, the plane performed unremarkably. By 1941 September, 39 sorties had made up 22 missions. Nearly half of the sorties were aborted due to mechanical problems. Eight of the twenty were destroyed by September, half to accidents. Their guns tended to freeze at high altitude and were generally unable to effectively protect the Fortresses. Their success as a bomber was also limited, largely because they were unable to hit anything from the altitudes at which they flew.

The first "C" series flew in July 1940; 38 were built. The eighteen remaining after twenty were transferred to the RAF were modified to the configuration used in the B-17D. However, one of these, B-17C 40-2047, crashed while being ferried from Salt Lake City, UT, to Mather Army Air Base, CA, on November 2, 1941.[2]

Though changes in the design made the Army Air Corps decide that the B-17D was worthy of a new subdesignation, the B-17C and B-17D were very similar. In fact, both were given the same subdesignation (299H) by Boeing.

Several minor changes were made, both internally and externally. Outside, the engines received a set of cowling flaps for better cooling, and the external bomb racks were removed. On the inside, the electrical system was revised, and another crew member added. In the dorsal turret and bathtub turret, the guns were doubled, bringing the total armament to one 0.30 in (7.62 mm) and six 0.50 in (12.7 mm) machine guns. The B-17D also featured more extensive armor protection. A total of 42 "D"s were built, and the 18 remaining B-17Cs were also converted to B-17D format. The sole surviving example of this model of the Flying Fortress - originally built in 1940 and nicknamed the Ole Betsy by its crew - is undergoing restoration at the National Museum of the United States Air Force in Dayton, Ohio. She was later renamed "The Swoose" by her pilot Frank Kurtz; he later named his daughter, actress Swoosie Kurtz, after the bomber.

The B-17E (299-O) was an extensive redesign of that used in previous models up to the B-17D. The most obvious change was a redesigned vertical stabilizer, originally developed for the Boeing 307 by George S. Schairer. The new fin had a distinctive shape for the time, with the other end of the fuselage retaining the well-framed, ten panel bombardier's nose glazing from the B-series design.

Because experience had shown that the plane would be vulnerable to attack from behind, both a tail gunner's position and powered fully traversable dorsal turret behind the cockpit, each armed with a pair of "light-barrel" Browning AN/M2 .50 cal. machine guns, were added to the B-17E design. Until this modification, crews had had to devise elaborate maneuvers to deal with a direct attack from behind, including jerking the aircraft laterally, allowing the waist gunners to alternate shots at enemy fighters. The configuration with 3-window box would also appear on the B-29, and also adopted by Soviet bombers as late as the Tupolev Tu-16 Badger, and in different form on the B-52. The teardrop-shaped sliding panels of the waist gunners were replaced by larger rectangular windows, directly across the fuselage from each other, for better visibility. In the initial fifth of the production run, the ventral bathtub gun emplacement of the C and D versions was replaced by a remote-sighted Bendix turret, very similar to the unit placed on the B-25B Mitchell medium bomber of the same period, which proved to be a disappointment in usability, resulting in the remaining E-series aircraft being fitted with a Sperryball turret, to be used for all succeeding B-17 versions.

Boeing B-17Es under construction. This is the first released wartime production photograph of Flying Fortress heavy bombers at one of the Boeing plants, at Seattle, Wash. Boeing exceeded its accelerated delivery schedules by 70% for the month of December 1942.

A total of 512 were built — possibly from the July 1940-dated order from the USAAC for B-17s being for that specific number of airframes[3] — making the B-17E the first mass-produced version of the B-17. One of these was later converted to the XB-38 Flying Fortress. Since production this size was too large for Boeing alone to handle, it was assisted by the Vega division of Lockheed and Douglas. Boeing also built a new plant, and Douglas added one specifically for production of the B-17.

In the middle of 1942, 45 B-17Es were transferred to the RAF, where they served under the designation Fortress IIA. Likely because of the shortcomings of the Fortress I (B-17C), the RAF decided not to use the Fortress IIA as a daylight high-altitude bomber, the role for which it had been designed. Rather, they were transferred to the Coastal Command for anti-submarine patrol.

Four known examples of B-17Es still exist in museums in the 21st century, none of which is currently known to be airworthy.

The B-17F was an upgrade of the B-17E, although outwardly the types were distinguished only by exchanging the bombardier's ten panel, well-framed nose glazing that had originated with the B-17B, for a molded one-piece or two-piece plexiglas cone, the two-piece version having a nearly-transparent diagonal seam. Late production "F" series aircraft received a "cheek"-mounted gun on each side of the nose, offset in their horizontal placing with the starboard "cheek" emplacement further forward, and fully feathering paddle-bladed propellers. Numerous internal changes were made to improve the effectiveness, range, and load capacity of the B-17. However, once placed in combat service, the "F" series was found to be tail heavy. The weight of gunners and ammunition when combat-loaded moved the center of gravity rearward from its design point and forced the constant use of elevatortrim tab, stressing this component. In combat the B-17F proved almost immediately to have inadequate defensive protection when attacked from the front. Various armament configurations of two to four flexible guns were utilized in the field, as sometimes cannibalized from damaged B-17s' tail gun positions (as one example), but the problem was not adequately addressed until the introduction of a powered, Bendix-designed remotely operated "chin" turret in the final production blocks of the F-series Fortresses — starting with the last 86 B-17Fs built by Douglas of the 605 B-17F-DL bombers built, from the B-17F-75-DL production block[4] — directly derived from its debut on the YB-40 experimental "gunship" version.

By using a stronger undercarriage, the maximum bomb capacity was increased from 4,200 lb (1,900 kg) to 8,000 lb (3,600 kg). Though this modification reduced cruise speed by 70 mph (110 km/h), the increase in bomb capacity was a decided advantage. A number of other modifications were made, including re-integrating external bomb racks, but because of its negative impact on both rate-of-climb and high altitude flight the configuration was rarely used and the racks were removed.

Range and combat radius were extended with the installation in mid-production of additional fuel cells in the wings. Called "Tokyo tanks", nine self-sealing rubber-composition tanks were mounted inside each wing on each side of the joint between the inner and outer wing sections. With an extra 1,080 US gal (4,100 l) to the 1,700 US gal (6,400 l) available on the first B-17Fs, the Tokyo tanks added approximately 900 mi (1,400 km) to the bomber's range.

3,405 were built: 2,300 by Boeing, 605 by Douglas, and 500 by Lockheed (Vega). These included the famous Memphis Belle. 19 were transferred to the RAF, where they served with RAF Coastal Command as the Fortress II. Three examples of the B-17F remain in existence in the 21st century, including the under-restoration Memphis Belle.

The most unusual conversion of the B-17G was the JB-17G converted to an engine test bed. The nose section was removed and replaced with a strengthened mount for a fifth engine.

Top view of a B-17G in flight

Generally considered the definitive B-17 design, all changes made in the B-17F production run were incorporated into the final version. These included a Bendix remotely operated chin turret, an innovation derived from the unsuccessful YB-40 escort version, bringing defensive armament to 13 .50 caliber (half-inch or 12.7 mm) machine guns, and for late production blocks, a revised tail gun position (referred to as the "Cheyenne" configuration after the modification center where it was introduced)[5] in which the guns were mounted in a remotely operated turret. Some 8,680 were built, and dozens were converted for several different uses:

CB-17G. Troop transport version, capable of carrying 64 troops.

DB-17G. Drone variant.

JB-17G. Engine test-bed.

QB-17L. Target drone.

QB-17N. Target drone.

RB-17G. Reconnaissance variant.

SB-17G. Rescue version, originally designated B-17H. Featured A-1 lifeboat under fuselage. After World War II, armament on the SB-17Gs was removed; it was reinstated when the Korean War began.

TB-17G. Special duty training version.

VB-17G. VIP transport.

PB-1. This designation was given to one B-17F and one B-17G. They were used by the U.S. Navy for various test projects.

PB-1G. This designation was given to 17 B-17Gs used by U.S. Coast Guard as air-sea rescue aircraft.

PB-1W. This designation was given to 31 B-17Gs used by the U.S. Navy as the first airborne early warning aircraft/ AWACS.

Eighty-five B-17Gs were transferred to the Royal Air Force, where they received the service name Fortress III. Three went to Coastal Command in the Azores fitted with radar before reuse with Meteorological squadrons. The rest were operated by two squadrons of Bomber Command's No. 100 Group RAF at RAF Sculthorpe from February 1944, where they were used to carry electronic countermeasures to confuse and jam enemy radar in support of bombing missions.[6] These Fortress III (SD)[7] would carry extensive array of equipment: the Monica tail-warning receiver, the Jostle VHF jammer,[8] Airborne Grocer air-interception jammers; Gee and LORAN for navigation and an H2S radar replacing the chin turret. They were also used as decoys during night bombing attacks. They took part in various such operations until the units were disbanded in July 1945.

The XB-38 was a modification project undertaken primarily by the Vega division of Lockheed on the ninth B-17E built. Its primary purpose was testing the feasibility of liquid-cooled Allison V-1710-89 engines. It was meant as an improved version of the B-17, and a variant that could be used if the Wright R-1820 engine became scarce. Completing the modifications took less than a year, and the XB-38 made its first flight on 19 May 1943. While it showed a slightly higher top speed, after a few flights it had to be grounded due to a problem with engine manifold joints leaking exhaust. Following the fixing of this problem, testing continued until the ninth flight on 16 June 1943. During this flight, the third (right inboard) engine caught fire, and the crew was forced to bail out. The XB-38 was destroyed and the project cancelled. The gains in modification were minimal and would have been disruptive to production of existing models. Allison engines were also considered to be more badly needed for constructing fighter aircraft.[9]

Prior to the introduction of the P-51 Mustang, a B-17 escort variant called the YB-40 was introduced. This aircraft differed from the standard B-17 in that a second dorsal turret was installed atop the radio operator's position between the forward dorsal turret and the waist guns, where only a single flexibly mounted Browning M2 gun had been; and the single 0.50 in (12.7 mm) machine gun at each waist station was replaced by a pair of 0.50 in (12.7 mm) guns, of basically the same twin-mount design as the tail guns used. In addition, the bombardier’s equipment was replaced with two 0.50 in (12.7 mm) machine guns in a remotely operated ‘chin’ turret to augment the existing ‘cheek’ machine guns, and the bomb bay itself was converted to a magazine. The YB-40 would provide a heavily armed "gunship" escort capable of accompanying the bombers all the way to the target and back. The aircraft was deemed a failure because it could not keep up with standard B-17Fs once they had dropped bombs. It was withdrawn from service after fourteen missions. (26 built: 1 XB-40 prototype, 21 YB-40 pre-production aircraft, 4 TB-40 training aircraft.)

Four B-17s were converted to serve as cargo carriers and V.I.P. transports under the designation C-108 Flying Fortress. (Many more served in the same roles under the designations CB-17 and VB-17, respectively.) The first of them, designated XC-108, was a B-17E partially stripped of military equipment and outfitted with various living accommodations. It served as a V.I.P. transport for General Douglas MacArthur. A similar conversion was made on a B-17F, which was redesignated YC-108. The third plane, designated XC-108A, was made to test the feasibility of converting obsolete bombers to cargo aircraft. B-17E 41-2595 was chosen for the conversion. Based in India, it ferried supplies over the Himalaya to the base for the B-29 Superfortress in Chengdu, China. It proved a difficult plane to maintain, due to lack of spare parts for the Cyclone engines, and was sent back to the United States, where it was based in Bangor, Maine, and flew a cargo route to Scotland until the end of the war. It was sold to a local dealer for scrap, but the airframe survived, and is currently being restored in Illinois. The final one was built under the designation XC-108B, and was used as a tanker to transport fuel from India to Chengdu.

Several B-17s were converted to long-range photographic reconnaissance aircraft, designated F-9 Flying Fortress. (The F- here stands for 'fotorecon' and must not be confused with F- for 'fighter', which was not introduced by the USAAF until after the war.)

The first F-9 aircraft were sixteen B-17Fs, with bombing equipment replaced by photographic equipment. Some of the defensive armament was kept. An uncertain number more were converted to a similar configuration to the F-9, but differed in minor details of their cameras, and received the designation F-9A. Some of these, along with more B-17Fs, received further camera alterations and became the F-9B. The last variant designation was the F-9C, which was given to ten B-17G, converted in a similar fashion to the previous planes. Those surviving in 1948 were at first redesignated RB-17G (R indicating 'reconnaissance').

Late in World War II, at least 25 B-17s were fitted with radio controls to be used as drones designated BQ-7 missiles, constructed under the auspices of Operation Aphrodite. Loaded with up to 20,000 lb (9,070 kg) of Torpex high explosive and enough fuel for a range of 350 mi (563 km) they were used to attack U-boat pens, V-1 missile sites, and other bomb-resistant fortifications.

The BQ-7s would be taken aloft by two volunteer crew members, who would take it up to 2,000 ft (610 m), point it toward the target, and transfer control to another B-17. They would then bail out through the open cockpit while still safely over England. The controlling B-17 would follow the BQ-7, and lock its controls into a collision course with its target, then turn around to escape.

Because remote-control hardware available at the time was insufficient for the task, Operation Aphrodite was riddled with problems. Between August 1944 and January 1945 15 BQ-7 were launched against Germany, none ever hit its target, and several crew were killed in various parachuting accidents. One bomber left a 100 ft (30 m) crater in British soil and another circled an English port out of control. It was cancelled in early 1945.

The U.S. Navy (USN) received 48 B-17's towards the end of World War II, renamed PB-1 and used for maritime patrol missions. Post-war, the USN acquired 31 more B-17G's, renamed PB-1W, and fitted with AN/APS-20 radar for Airborne Early Warning equipment and procedure development.

The Naval Air Material Center’s Naval Aircraft Modification Unit (NAMU) at Johnsville, Pennsylvania modified the B-17s to PB-1W specification by sealing up the bomb bay doors and installing 300 gallon drop tanks on each wing, in addition to the “Tokyo Tanks” mounted in the outer wings, holding a total of 3,400 gallons of fuel, giving the PB-1W an endurance of 22+ hours. Initially PB-1W's retained the natural metal finish with a protective wax coat, but later the PB-1Ws were painted gloss Navy Blue overall.

From 1943 to 1948, as part of Dumbo missions, 12 B-17G's were converted to B-17H's equipped with an airborne lifeboat and ASV radar for USAAF air-sea rescue duties. The US Coast Guard flew 17 similar aircraft as PB-1G's.